1. Field of the Invention
The present invention relates to a technology for adjusting an optical level of a transmission signal to an optimum level in an optical transmission system.
2. Description of the Related Art
Recently, the optical transmission system for performing transmission of optical signals with a high transmission rate by using an optical fiber for a transmission path, multiplexed by wavelength division multiplexing (WDM), and capable of increasing the information capacity has been popularized and used, instead of electric signals. FIG. 9 is an explanatory diagram of a configuration example of the optical transmission system.
In an optical transmission system 900, optical add-and-drop multiplexers (OADMs) A, B, E, and D, and in-line amps (ILAs) C and F are provided on a transmission path including an outer ring (upward ring) 910 and an inner ring (downward ring) 920. Transceivers 901A, 901B, 901D and 901E are connected to the OADMs A, B, D, and E, respectively, and transmission and reception of optical signals can be performed with an optional communication partner, by adding, dropping, or transmitting transmission light transmitted through the outer ring 910 and the inner ring 920. The ILAs C and F amplify a WDM beam transmitted through the outer ring 910 and the inner ring 920. The light transmitted in the optical transmission system 900 is formed of the WDM beam obtained by multiplexing the optical signal and an optical supervisory channel (OSC) beam for supervising the transmission state of the optical signal.
In the optical transmission system 900, it is important to adjust the optical level of the optical signal constituting the WDM beam to an appropriate value by the OADMs A, B, D, and E and the ILAs C and F to transmit the optical signal through the outer ring 910 and the inner ring 920.
As the conventional art relating to the adjustment of the optical level, there is a structure in which in the wavelength multiplexing optical transmission, substantially equal optical output can be obtained in each wavelength, thereby enabling insertion of an optical functional part into an intermediate portion, regardless of the level and the wavelength of the optical signal input to an optical fiber amplifier. In this case, it is important to avoid occurrence of optical surge and determine a connection of parts. Therefore, a technique is disclosed in which feedback control is performed by inserting a variable attenuator in an optical input unit, so that the optical input to the amplifying optical fiber becomes constant. Furthermore, control for changing the overall optical output and optical input to the amplifying optical fiber is performed based on the wavelength information obtained from a supervisory signal, and light to the intermediate optical part and light from the optical part are detected, and when there is no part, pumping is suppressed. By performing such control, occurrence of optical surge at the time of connection can be avoided, and a signal indicating that an optical part is not connected is output (see, for example, Japanese Patent Application Laid-Open No. H11-17259).
There is another example in which an optical wavelength multiplexing network can be easily formed. In this technique, it is important to keep constant an optical signal level for each channel, to maintain desired transmission quality. Therefore, a supervisory signal transmitted through the optical fiber transmission path is extracted by a WDM coupler, to obtain the wavelength of the optical signal input to a remote node from the supervisory signal. A feedback controller calculates the wavelength information, which is the sum of the wavelength obtained from the supervisory signal and the wavelength of an optical signal newly added at the remote node, via a supervisory signal processing circuit. Furthermore, by adjusting an attenuation of the variable optical attenuator so that a value obtained by dividing the total optical power of an optical amplifier by the value of wavelength becomes the desired optical power of the optical signal for each channel, feedback control is performed at all times with respect to the attenuation of the variable optical attenuator, to compensate loss fluctuation in the optical fiber transmission path (see, for example, Japanese Patent Application Laid-Open No. 2004-147122).
Conventionally, the control of the optical level of the optical signal is performed at the time of startup of the optical transmission system, as in Japanese Patent Application Laid-Open Nos. H11-17259 and 2004-147122. The attenuation of a reception unit is adjusted to control to the optical level to an optimum level, based on the wavelength information of the WDM beam obtained by the OSC controller equipped in the OADMs A, B, D, and E and the ILAs C and F shown in FIG. 9.
An example of the method of adjusting the optical signal level at the time of startup (activation) of the OADM or the ILA is shown below. FIG. 10 is an explanatory diagram of a startup procedure of the optical transmission system. A reception unit 1010 includes a variable optical attenuator (VOA) 1011, a front photodiode (PD) 1014 arranged upstream of the VOA 1011, a rear PD 1015 arranged downstream of the VOA 1011, an OSC branch coupler 1012, and a preamp 1013. A transmission unit 1050 includes a postamp 1051, an OSC combination coupler 1052, and a 1×2 switch (SW) 1054. The reception unit 1010 and the transmission unit 1050 further include unit controllers 1016 and 1053, respectively. The unit controller 1016 of the reception unit 1010 adjusts the attenuation of the VOA 1011 based on optical levels detected by the front PD 1014 and the rear PD 1015, to control the optical level of the optical signal input to the preamp 1013. The unit controller 1016 of the reception unit 1010 and the unit controller 1053 of the transmission unit 1050 are connected to an OSC controller 1060 (for convenience, it is written as “OSC” in the drawings, as well as an OSC controller explained below), to adjust the attenuation of the VOA 1011 at the time of startup.
An OR 1061 and an OS 1062 includes a unit controller 1063, an optoelectronic converter (OE) 1064, and an electro-optic converter (EO) 1065. The unit controller 1063 controls the OSC controller 1060. The OE 1064 converts an input optical signal to an electric signal and output the electric signal. The EO 1065 converts an input electric signal to an optical signal and output the optical signal.
The startup procedure of the OADM B connected to the outer ring 910 and the inner ring 920 is explained next. The startup of the OADM B is performed by transmitting the OSC beam between adjacent optical transmission devices (that is, OADMs A and B in the example shown in FIG. 10).
At first, an output request of amplified spontaneous emission (ASE) beam for optical level control is output from the unit controller 1063 in the OSC controller 1060 of the OADM B to the unit controller 1016 of the OADM B and the unit controller 1016 of the OADM A (S1). The optical level of the ASE beam requested at this time corresponds to one wavelength level of the optical signal. In response to the output request of the ASE beam, a 1×2 switch (SW) 1017 arranged upstream of the preamp 1013 in the OADM B is controlled to open, so that the optical signal from the OADM B is not sent out to the transmission path, thereby shutting down the input light to the OADM B.
Subsequently, communication confirmation of the OSC beam is performed in the EO 1065 of the OADM A and the OE 1064 of the OADM B (S2). The postamp 1051 having received the output request of the ASE beam outputs the ASE beam of a level corresponding to one wavelength of the optical signal (S3). At this time, a 1×2 SW 1054 arranged upstream of the postamp 1051 in the OADM A is controlled to open.
When the ASE beam is input to the reception unit 1010 of the OADM B via the outer ring 910 (S4), and further input to the unit controller 1016 via the VOA 1011, auto-adjustment of the VOA 1011 is carried out (S5). Specifically, to make the input light of the preamp 1013 at an appropriate level, the unit controller 1016 in the OADM B monitors the light-receiving power of the rear PD 1015 arranged upstream of the preamp 1013, and adjust the VOA 1011 to have an appropriate attenuation.
When the auto-adjustment of the VOA 1011 has finished, the unit controller 1016 in the OADM B determines that the input to the preamp 1013 becomes stable, to release the shut-down state of the preamp 1013 in the OADM B (S6), and starts up the preamp 1013 by automatic level control (ALC).
When having confirmed that the preamp 1013 has been started up, and shifted to automatic gain control (AGC), the unit controller 1016 in the OADM B suspends the output request of the ASE beam for optical level control from the unit controller 1063 (S7). When the output of the ASE beam from the postamp 1051 has stopped, the unit controller 1053 closes the 1×2 SW 1054 arranged upstream of the postamp 1051 in the OADM A, to release the shut-down state of the postamp 1051, and starts the operation thereof.
The auto-adjustment of the VOA 1011 carried out at S5 in FIG. 10 indicates a process for adjusting the optical level of the optical signal input to the preamp 1013 (the ASE beam at the time of startup) to be within a dynamic range of the preamp 1013.
After the startup operation as described above, the OADM A and the OADM B are in a normal operation state. The VOA 1011 fixes the attenuation for one wavelength of the optical signal, and the preamp 1013 carries out automatic gain control (AGC) to control the gain of the multiplexed optical signal to be equalized. This is because in the optical transmission system 900, it is assumed that the wavelengths of optical signals multiplexed in the WDM beam on the transmission path changes corresponding to the communication state. Therefore, even at the time of increase or decrease in the wavelengths of optical signals, the OADMs A and B can keep the level of the optical signal at an appropriate level.
However, even in the optical transmission system 900 in which optical level control is carried out by the OADMs A, B, D, and E and the ILAs C and F, if bending or an excessive temperature change occurs in the transmission path (for example, the outer ring 910 or the inner ring 920) itself, the transmission characteristic of the transmission path changes, thereby affecting the optical level of the WDM beam. When the transmission characteristic has changed, the attenuation fixed at the time of startup is attenuated as usual in the VOA 1011, since there is no change in the wavelengths of the optical signals multiplexed in the WDM beam.
As a result, the WDM beam, whose optical level has changed as compared with the optical level at the time of startup or at the time of normal operation, due to a change in the transmission characteristic of the transmission path, is input to the OADMs A, B, D, and E and the ILAs C and F. Such a WDM beam is output directly to the transmission path, with the change in the optical level being not corrected. When the optical level changes of the WDM beam are accumulated, the changes cannot fall within the dynamic range of the pre-designed input level to the OADMs A, B, D, and E and the ILAs C and F, thereby causing an error.
When the dynamic range of the input level is designed to be large, taking the changes in the transmission characteristic into consideration, the production cost of the OADMs A, B, D, and E and the ILAs C and F increases.